1. Field of the Invention
This invention relates to furnaces for heat treating a running length of metal passing therethrough and more particularly to an improved electric induction furnace particularly suited for heat treating a coated metal strip such as zinc coated steel strip following application of the coating, for example in a hot dip galvanizing operation.
2. Description of the Prior Art
In the production of coated steel product in which a steel substrate is coated with an iron zinc alloy, it is known to initially apply the coating as a substantially iron-free zinc coating and to alloy the zinc in the coating with iron from the underlying substrate by heating the coated substrate to a temperature and for a time to permit iron molecules from the steel substrate to migrate through the zinc to produce the desired alloy coating.
Heat necessary to alloy a zinc coating on a continuous ferrous substrate such as steel strip or wire (hereinafter strip) is conventionally applied by passing the zinc coated strip in a running length through a furnace similar to that employed in a continuous annealing operation. Such furnaces normally have an elongated heating chamber through which the running length passes and in which heat is applied in a controlled manner to quickly heat the running length to the desired temperature and to maintain it at this temperature for the desired length of time. Conversion of an essentially pure zinc coating, or a coating containing small amounts of aluminum, to a zinc-iron alloy coating in this manner has become known in the industry as a galvanneal process and the product produced is conventionally referred to as a galvanneal product. The term "galvanneal" will therefore sometimes be used herein to refer generally to this basic process and product.
Examples of prior art apparatus and process employed in the production of galvanneal, and particularly for the production of iron zinc alloy coated steel in continuous strip form can be found in U.S. Pat. Nos. 2,986,808 to Schnedler, 3,056,694 to Mehler et al and 3,190,768 to Wright. Each of these prior art patents discloses the coating of a continuous or running length of steel strip in a conventional hot dip galvanizing operation wherein the strip material receives suitable pretreatment to remove oxides, rolling oils and the like from its surface and is then conducted through a controlled atmosphere into a spelter pot containing a supply of molten zinc. From the spelter pot, the zinc coated strip passes upwardly and immediately into the galvanneal furnace, preferably while the zinc coating is still in the molten state, where heat is applied to increase the temperature of the zinc coating and the underlying steel substrate to a temperature and for a sufficient time to permit alloying of the coating with the iron molecules from the steel substrate. A similar process is described in "Making, Shaping and Treating of Steel", 8th Ed., pg. 987, wherein it is stated that the galvanneal coatings have a silvery matte finish having good paint adherance properties without requiring further surface preparation and are capable of withstanding moderate forming operations. It will be recognized that various factors including strip thickness, coating weight, the speed of travel of the strip through the furnace and the length of the furnace chamber will determine the length of time that the strip is exposed to the heat treatment process.
While galvanneal has been known and commercially used for many years, difficulty has been encountered in the production of a satisfactory product for many uses at a commercially acceptable rate. For example, minor variations in the thickness of the zinc coating applied to a steel strip as well as variations in the steel substrate itself, including surface conditions, strip thickness and the like, can result in non-uniform alloying of the zinc while passing through the galvanneal furnace. In addition, the furnaces conventionally used in the production of galvanneal have not been entirely satisfactory in that they generally have not been capable of uniformly heating the coated steel or maintaining the desired temperature throughout the substantially straight path through the furnace.
Although the use of electric induction heating is suggested by each of the above-mentioned U.S. patents, in commercial practice gas fired furnaces of the general type described in the Mehler et al patent have been used for the continuous production of galvanneal in strip or strand form. Such gas fired furnaces, when employed for the production of galvanneal in a continuous strip coating line, have included banks of gas fired burners positioned on both sides of the strip, with the banks on each side being enclosed in an insulated box-like housing. Such furnaces inherently produce large volumes of high temperature gas which, combined with air drawn by convection into the bottom of the furnace, passed upward with the moving strip in its path through the furnace. As indicated in the Mehler et al patent, the primary concern in such gas fired furnaces is to provide adequate space between the jets and the moving strip so that the jets do not impinge too closely onto the surface of the strip to thereby avoid disturbance of the molten metal coating and to avoid localized hot spots. The size and construction have made it impossible or impractical to accurately control the position of the jets relative to the strip in the known gas fired galvanneal furnaces commercially available.
It is known to mount gas fired galvanneal furnaces for movement along horizontal tracks extending perpendicular to the direction of movement of the strip and parallel to the side surfaces of a strip moving through the furnace. The separate banks of gas jets on each side of the furnace enabled the box-like housing to be constructed with one end open to permit movement along these tracks for removal of the furnace from the strip processing line, as for example, when ordinary hot dip galvanized strip was to be produced on the line.
Other problems inherent in the commercial gas fired galvanneal furnaces are discussed in the above-mentioned Schneider patent which is directed to an electric heating furnace for the production of galvanneal. This prior art patent teaches that either resistance or induction heating may be employed and two versions of an induction heating furnace are described. In one version, two banks of induction coils are disposed one on each side of a strip moving through the furnace much in the same manner as the banks of gas jets in a conventional gas fired galvanneal furnace. In the other version, the induction coils extend completely around the strip. Experimental versions of this heating arrangement only are described, however, and to the best of applicant's knowledge, a commercially acceptable furnace for use in the high speed production of galvanneal strip was not developed for utilizing the inventive concept of this patent.
The arrangement of FIG. 3 of the Schnedler patent wherein a plurality of coils are disposed on each side of the strip would inherently result in a substantially less efficient utilization of the electrical energy. While the arrangement of FIG. 2 would be more efficient, this arrangement results in a furnace construction wherein the strip would necessarily have to be cut in order to move the furnace from the position above the zinc pot, making it necessary to rethread the system if such a furnace were constructed for movement in the manner described above with respect to gas fired furnaces. In this respect, it is noted that all of the prior art patents mentioned above are totally silent as to construction of the furnace and do not suggest any means for control to maximize efficiency of the units or to enable accurate positioning of the induction units relative to the strip passing therethrough.